Country Rocks Research Articles

Magmatic to solid-state evolution of a shallow emplaced agpaitic tinguaite (the Suc de Sara dyke, Velay volcanic province, France): implications for peralkaline melt segregation and extraction in ascending magmas

Abstract. In the last decades the mush model has been generalized to the complete trans-crustal magmatic system in which differentiation would be driven by segregation and extraction of trapped melts from crystal-rich mushes. Melt extraction processes involved are porous flow and strain localization, the latter being regarded as the main process acting during transfer through dykes and necks along which high differential stresses are acting on. We combine structural measurements together with petrological analyses and textural observations to constrain the model of emplacement and finally emphasize how shear deformation and strain localization structures promoted the residual melt segregation that occurred in a shallow silica-undersaturated peralkaline intrusion (Suc de Sara, Velay volcanic province, French Massif Central). In this study, we demonstrate that segregation and subsequent extraction of the CO2-rich residual melt occurred during magma ascent and final emplacement of the Suc de Sara tinguaite. Contrasting features of shear deformation between the margins that exhibited different permeabilities highlight that melt segregation started by compaction as a loose packing of emerging microlites and continued with melt filling of an anastomosed C/C′ band network developing in the crystal-rich mush subjected to high shear strain. Subsequent melt extraction throughout the country rock was controlled by the permeability of the hanging wall. Along the western hanging wall of the intrusion, extraction of the residual melt was prevented by the 15 cm thick chilled margin. In contrast, segregated melt circulated through the highly porous and permeable eastern margin, causing the fenitization of the country rock.

Gold content determination in small core-samples by instrumental neutron activation analysis

Application of instrumental neutron activation analysis (INAA) based on the relative method of concentration standardization was considered to determine Au content in small solid core-samples of the gold-bearing ores and rocks up to 10 g of the mass. The small core-samples about 10 mm in diameter and 22-23 mm in length were cut from the previously collected ore lumps or in-situ using a handheld drilling rig. The studied small core-samples and the polyethylene capsules of the same dimensions filled up with the corresponding certified reference material (CRM) were irradiated for 2 min by a lower density neutron flux with the help of the automated pneumatic transport system (PTS). Maximum mass and dimensions of the core-samples corresponded to the PTS design and to the conditions of its safe operation. A special device was made to fix the transport capsules in the stable counting geometry making possible to eliminate the influence of the neutron flux gradient during irradiation. Due to the substantial differences in CRM and solid sample densities, corrections for neutron self-shielding and for gamma-ray self-absorption by the core-samples were applied. The method was tried to analyze Au content in 170 small core-samples presenting different gold-bearing ores and country rocks from the Kazakhstan’s gold-barite-polymetallic deposit Maikain. This approach, rather simple methodically and requiring no unique equipment, can be used to assess gold resources together with the methods of geostatistics.

Amphibole geochemistry in the Haobugao skarn Zn-Fe-Sn deposit in the southern Great Xing’an Range: Implications for the ore-forming process

The Haobugao Zn-Fe-Sn deposit is a typical skarn deposit in the southern Great Xing’an Range of NE China. Despite many advances in understanding its genesis, the detailed ore-forming processes of Zn, Fe and Sn are still unclear. Amphibole, a rock-forming mineral, has been well studied as an indicator of diverse magmatic and metamorphic processes; however, the geochemistry of hydrothermal amphibole has previously received less attention. The Haobugao deposit contains two generations of hydrothermal amphibole (Amp-I and Amp-Ⅱ), which provides a good opportunity to use amphibole geochemistry to trace the mineralization process. Early Amp-I coexists with cassiterite, magnetite and quartz, whereas late Amp-II coexists with quartz and sulfides and can be further divided into two subgroups: Amp-Ⅱa (in skarn) and Amp-Ⅱb (in the country rocks of siltstone). The oxygen isotopic compositions of the fluid (δ18Ofluid 2.9‰) responsible for the Amp-I formation indicate that the amphiboles formed from mixed magmatic fluid with meteoric water. Amp-I is LREE-depleted and HREE-enriched with obvious negative Eu anomalies. Amp-Ⅱ is generally slightly LREE-depleted with negative or positive Eu anomalies and positive Ce anomalies. The incorporation of trace and rare elements is mainly controlled by the fluid composition. Detailed petrological observations and amphibole geochemistry, combined with previous research results, confirm that Sn, Fe, Pb and Zn mineralization occurred during a single mineralization event. From the oxide to the quartz sulfide stage, the ore-forming fluid evolved from high-temperature and oxidized conditions with enrichment of Sn, Li, Be and REEs to low-temperature and reduced or oxidized conditions with enrichment of Cu, Co and Zn. The concentrations of Cu, Co and Zn in different amphiboles vary with the mineral crystallization sequence. This study shows that amphibole geochemistry can be useful for tracing fluid evolution and mineralization processes in hydrothermal systems.

Evidence for the formation of silicic lava by pyroclast sintering

Silicic lavas can be produced by the sintering of pyroclasts in the volcanic sub-surface, and then advected out of the vent. Here, we provide evidence for this mechanism preserved in the exposed post-glacial remnants of a silicic volcanic conduit at Hrafntinnuhryggur, Krafla volcano, Iceland. We show that the conduit margins are a clast-supported pumice lapilli tuff deposit that grades continuously into dense obsidian and that the obsidian contains cuspate relict clast boundaries and country rock lithic fragments throughout. Transects of H2O concentrations across the conduit show that the magma was degassed to different degrees laterally with systematic spatial variation that is consistent with progressive conduit clogging and final gas pressurisation. Textures in the overlying effusive lavas record the variably sheared and brecciated remnant of the same in-conduit sintering. This record of a silicic conduit system connected to upper eruptive deposits provides support for the ‘cryptic fragmentation model’ for effusive silicic volcanism.

The newly discovered Attu carbonatite of West Greenland: A Mesoproterozoic dyke intrusion enriched in primary Sr-Ba-REE minerals

A newly discovered ca. 1.5 Ga old carbonatite dyke in central West Greenland is characterized by very high contents of rare earth elements (REE), Sr and Ba (up to 12.4 wt% total REE, 14.6 wt% SrO and 12.4 wt% BaO). The dyke occurrence is named ‘Attu carbonatite’ after the nearby village Attu. The carbonatite is primarily composed of carbonate minerals such as Sr-rich calcite (CaCO3) and dolomite (CaMg(CO3)2), huntite (Mg3Ca(CO3)4), strontianite (SrCO3), alstonite (CaBa(CO3)2), burbankite ((Na,Ca)3(Sr,Ba, Ce)3(CO3)5) and daqingshanite ((Sr,Ca,Ba)3(Ce, La)(PO4)(CO3)3−x(OH, F)x). In addition to the wide range of carbonate minerals, the carbonatite contains coarse-grained monazite(Ce), apatite and magnetite. Barite occurs as discrete crystals together with the rock-forming carbonates. Texturally, the carbonatite rock displays abundant intimately intergrown fine- to medium-grained Ca, Sr, Ba and REE carbonate minerals, which exhibit prominent exsolution textures within calcite, burbankite, strontianite and dolomite hosts, as well as in the apatite crystals. Several different exsolution textures are observed: 1) alstonite in calcite; 2) daqingshanite in calcite; 3) daqingshanite in burbankite; 4) MgBa carbonate in strontianite; 5) MgBa carbonate in calcite and strontianite in dolomite; 6) strontianite in apatite; and 7) monazite(Ce) in apatite. The carbonatite dyke is foliated and exsolution textures are observed internally in the foliation-defining minerals indicating that exsolution occurred after the main deformation event.The carbonatite magma intruded into Archaean basement gneisses that had been affected by the Nagssugtoqidian tectono-metamorphic event at approximately 1850 Ma. Magmatic monazite(Ce) crystals from the carbonatite yield a UPb age of 1565 ± 53 Ma, which is the current best estimate of dyke emplacement. Monazite(Ce) that exsolved from apatite and calcite yields a UPb age of 1492 ± 33 Ma, which is within the analytical uncertainty of the primary magmatic monazite UPb age. However, the UPb age determinations suggest that mineral exsolution occurred a few million years after carbonatite magma emplacement, in response to further cooling of the deep-seated dyke (tectonically-induced uplift?). Dyke emplacement may have occurred within an active ductile shear zone, which helps to explain the foliation of the carbonatite rock, predating cooling-related mineral exsolution. Country rock fenitization by fluids that emanated from the carbonatite dyke intrusion is recorded by the increasing abundance of mafic silicates such as Ba-rich phlogopite at the contact zone.

Contributing to understand how Cordilleran batholiths build from detailed geochronology of the Carboniferous Tabaquito batholith, Frontal Cordillera (Argentina)

Cordilleran Granitic batholiths or Andean/Cordilleran batholiths serve as plutonic expressions of continental arcs, offering valuable insights into the processes that operate in large silicic magmatic systems at subduction environments. This work presents a comprehensive geochronological study of the Carboniferous Tabaquito batholith in the Frontal Cordillera of western Argentina, which is the best exponent of the Carboniferous arc-related magmatism in this region. Using new data obtained through LA-MC-ICP-MS and comparing it with previously published data (LA-MC-ICP-MS and SHRIMP techniques), we identify five distinct magmatic events in the Tabaquito batholith: 325 ± 2 Ma; 332 ± 2 Ma; 337 ± 2 Ma; 346 ± 1 Ma, and 362 ± 2 Ma. The 325 Ma event is considered as the best estimate of the crystallization age during the emplacement. The 332 Ma, 337 Ma, 346 Ma and 362 Ma events suggest the presence of zircon antecrysts, implying a complex magmatic system that worked for a protracted time lapse of ca. 40 Myr, or ca. 15 Myr if we consider the ages of 346 and 362 Ma as xenocrysts from a source like the Potrerillos pluton (346 ± 3 Ma to 356 ± 3 Ma), which was interpreted in previous studies as possible source of the parental magmas of the Tabaquito batholith. Furthermore, it has been also identified inherited zircons with ages older than ca. 370 Ma that are consistent with the detrital zircon pattern obtained for the country rock. Inherited zircon ages from the referred country rock yield a maximum deposition age estimated at 387 Ma that is consistent with the fossil ammonoids record, indicating that the granitic rocks were emplaced in Devonian rocks.

Hydrous shear zones are sites of melt transfer in the lower arc crust: A case study from Fiordland, New Zealand

AbstractRecent studies of the Cretaceous lower arc crust exposed in Fiordland, New Zealand, conclude that shear zones are sites of melt migration and mass transfer through the deep crust. Here, we investigate the 4–10 km‐wide George Sound Shear Zone, which cuts the Western Fiordland Orthogneiss, comprising two main rock types: two‐pyroxene gneiss and hornblende gneiss. Previous studies infer a predominantly igneous origin for the two types of gneiss. However, this study finds that melt‐rock interaction within the George Sound Shear Zone formed the hornblende gneiss from the precursor two‐pyroxene gneiss. Petrographic analyses of samples collected in transects across the shear zone show hydration reaction textures ranging from rims of hornblende + quartz around pyroxene grains to complete replacement of pyroxene grains. Plagioclase is recrystallized and partially replaced by clinozoisite. Additionally, biotite mode increases towards the higher strain rocks in the shear zone. Backscatter images and polarized light microscopy show microstructures indicative of former melt‐present deformation, including (a) interconnected mineral films of quartz and K‐feldspar along grain boundaries, (b) grains that terminate with low apparent dihedral angles, (c) interstitial grains, with some (d) undulose extinction in plagioclase and (e) serrated grain boundaries. In addition, zircon microstructures are consistent with Zr mobility, further supporting the former presence of melt; geochemical data show enrichment of Zr in the hornblende gneiss as compared to the two‐pyroxene gneiss. From the above observations, it is inferred that a felsic to intermediate hydrous melt migrated through the George Sound Shear Zone reacting with the host two‐pyroxene gneiss of the Western Fiordland Orthogneiss. Melt migration along grain boundaries was deformation assisted, (i) causing hydration of pyroxene to hornblende + quartz, and plagioclase to clinozoisite, (ii) increasing proportions of biotite within the shear zone and (iii) causing depletion of Cr + Ni and Zr enrichment in the hydrated rock. Our interpretation is supported by published observations of pegmatite dyke swarms that intruded into the George Sound Shear Zone, the P‐T conditions of deformation and characterization of microstructures that contrast sharply with those typically found in mylonitic rocks formed under solid‐state metamorphic conditions. Our results confirm that hydrous shear zones within otherwise anhydrous country rock are retrogressive and may represent evidence of melt migration through zones of deformation.

Late Neoproterozoic tholeiitic plutonism in far-east Avalonia (the Dirgine Batholith, Istanbul Zone): Exposed mid-crustal section of an oceanic arc

Avalonia, a large composite terrane, extends from the northeastern regions of America and Canada to southern Britain and Belgium/Netherlands, continuing on to Poland. It was accreted to Laurentia and Baltica during early Paleozoic. The late Neoproterozoic basement of the Istanbul Zone (NW Turkey), which is considered the far-east extension of Avalonia, is poorly known. This study deals with the petrology and age of the Dirgine Batholith, the largest component of the late Neoproterozoic basement. The batholith, ∼70 km long and ∼12 km wide, was emplaced into Neoproterozoic low-grade mafic to felsic metavolcanic and metapyroclastic rocks. Tonalite and quartz diorite form the bulk of the batholith. Trondhjemite, granodiorite, and anorthositic gabbro occur in minor amounts. Overall, rock types resemble low-Al tonalite-trondhjemite-granodiorite series and trondhjemites in ophiolite sequences. The batholith formed over a narrow time interval (562–574 Ma, late Ediacaran), as inferred by U–Pb zircon geochronology. In-situ Lu-Hf analyses of igneous zircons reveal a limited range of significantly positive initial εHf values (9.47 ± 1.46, 2σ) and young Hf depleted mantle model ages (0.78 ± 0.06 Ga), indicating the relatively juvenile character of original magmas. All rock types have typical subduction-related trace-element signatures such as depletion in high-field-strength elements and enrichment in several fluid-mobile elements as well as nearly flat chondrite-normalized REE patterns. Geochemical characteristics indicate that crustal assimilation was insignificant, and the batholith formed by fractional crystallization of mantle-derived low-K tholeiitic magmas involving mainly clinopyroxene, hornblende and ± plagioclase. The Dirgine Batholith and metamorphic country rocks probably represent the mid-crustal section of a late Neoproterozoic oceanic arc. The interpretation of an oceanic rather than continental arc is based on the significantly lower concentrations of incompatible elements relative to continental arcs, nearly flat REE patterns of the rock types, and the juvenile nature of the primary magmas. These data, in concert with those in the literature, reveal that late Neoproterozoic oceanic arcs constitute an important component of far-east Avalonian terranes.

How old is the Eye of Africa? A polyphase history for the igneous Richat Structure, Mauritania

The annular Richat Structure, in Mauritania, is among the most striking geological features on Earth visible from space. Nicknamed “the Eye of Africa”, its intriguing concentric shaping has drawn attention for several decades. Formerly hypothesized as an astrobleme, it is now consensually recognized as a complex igneous intrusion, involving two ring-like gabbroic bodies, a central breccia and carbonatite dykes. However, the coexistence of until now undated tholeiitic gabbros and 99 Ma carbonatites akin to alkaline magmatism remained enigmatic.We provide the first plagioclase 40Ar/39Ar age determination and new geochemical data on the gabbroic bodies. Although no robust age was obtained, numerical modelling of our results suggests that the gabbros were intruded between 230 and 200 Ma. This age bracket is compatible with the ∼200 Ma Central Atlantic Magmatic Province (CAMP). Moreover, the compositions of these tholeiitic gabbros match the two chemical groups of the CAMP the best represented in northwest Africa. Therefore, we argue that the Richat gabbros correspond to two CAMP sills intruded conformably into the Late Proterozoic/Lower Paleozoic sedimentary sequences of the Taoudenni basin. About 100 My later, the gabbros and their country rocks were locally uplifted by an alkaline intrusion (expressed now on surface by the carbonatites), resulting in a circular doming, 40 km in diameter. The subsequent erosion led to the actual flattened concentric structure. Therefore, the Richat Structure represents a two-stage igneous-history, separated by a time lapse of ∼100 My, resulting in two CAMP gabbroic sills mimicking ring-like bodies.

Geology, geochronology and tectonic context of the Carreira Comprida Anorthosite in the evolution of the Goiás Massif, Porto Nacional Domain – Tocantins Province

The Carreira Comprida Anorthosite (CCA) is exposed in the Paleoproterozoic Porto Nacional High-Grade Metamorphic Terrene, in the north of the Goiás Massif, central Brazil, represented by an ancient pluton affected by late Neoproterozoic tectonics, with protoliths predominantly of anorthosite consisting of more than 85% labradorite (An60-65), with subordinate presence of clinopyroxene, quartz, hornblende and garnet. There is evidence of intrusive relationships in paragneisses country rocks of the Morro do Aquiles Formation, due to the effects of high-temperature contact metamorphism (∼750 °C) that reached the hornblende hornfels facies. Features of a magmatic nature are present as a cumulate texture formed by the accumulation of tabular plagioclase megacrystals by flotation associated with gravitational action due to the difference in magma density and magmatic flow, with clinopyroxene and interstitial (intercumulated) quartz, during the emplacement of the pluton and crystallization of magma. At the end of the Neoproterozoic, the body was affected by the tectonics of the transcontinental strike-slip system of the Transbrasiliano Lineament (Porto Nacional Transcurrent Shear Zone). U–Pb dating on igneous zircon crystals indicated an age of 2084 ± 4 Ma, which represents the age of emplacement of the Carreira Comprida Anorthosite in the Rhyacian, characterizing an event of gabbroic-anorthosite magmatism not known in the Goiano Massif. Lu–Hf isotopic data show variable εHf (2.08 Ga) values ranging from −1.3 to +4.5 and Hf-TDMC model ages with two crust formation episodes, one from the Siderian (2.46 - 2.38 Ga) and another in Neoarchean (2.74 - 2.59 Ga) which indicates that their parental magmas were derived from a depleted mantle source and range in composition from uncontaminated to variably contaminated by pre-existing older crust in the formation of the CCA. Considering that the ages of the Hf-TDMC model are similar to those of the orthogranulites from the Porto Nacional Complex, its indication as a possible source of the CCA is suggestive. Field relationships, primary structural features, petrographic and compositional data, geochronological and literature data suggest that the emplacement of the CCA is late-tectonic in relation to the evolution of the high-grade terrain of Porto Nacional, in the Rhyacian. After its crystallization, the CCA was affected by the tectonics of the dextral transcurrent system of the Transbrasiliano Lineament that implemented the Porto Nacional Shear Zone at the end of the Neoproterozoic, imposing strong deformation on the anorthosite and surrounding rocks, generating a variety of mylonites. These transformations generated a diversity of microstructures (stretch lineation, mylonitic foliation, tectonic banding, ribbon quartz, lentiform porphyroclasts, microshear, undulatory extinction, thinning of mechanical twinning, wedge twinning, deformation bands, mantle-core, subgrains, symplectites of garnet), which are suggestive of ductile tectono-metamorphic transformations that reached amphibolite facies conditions. Finally, the CCA is a rare example in Brazil of a massif-type anorthosite developed in the Paleoproterozoic (2.08 Ga). On the other hand, the Rhyacian granite bodies of the Ipueiras Suite (2.08 - 2.07 Ga) located very close to the anorthosite pluton are contemporary with the CCA, allowing them to be considered an association of the Anorthosite-Mangerite-Charnockite (AMC) type.

New metamorphic constraints on the Nova-Bollinger Ni–Cu deposit, Fraser Zone, Western Australia

Metamorphic investigations were conducted using mineral chemistry, phase equilibria modelling and conventional thermobarometry on four metamorphic country rock samples from the Nova-Bollinger Ni–Cu deposit, Western Australia. New P–T constraints obtained from phase equilibria modelling of garnet-bearing granulites and cordierite–sillimanite-bearing metasedimentary rocks indicate that the Nova-Bollinger deposit formed at mid-crustal depths (0.75–0.76 GPa and 880–920 °C), along high thermal gradients around 1200 °C/GPa. Peak metamorphic conditions are consistent with those obtained from the Nova-Bollinger intrusive rocks, and with metagabbros from the southern Fraser Zone, but are slightly elevated relative to metapelites examined in the southern Fraser Zone. The peak metamorphic conditions at Nova-Bollinger reflect the combined effects of regional-scale high-T conditions during orogenesis superimposed by contact metamorphism in a thermal aureole adjacent to the Nova-Bollinger mafic–ultramafic magmas. Thermal metamorphism may have been further enhanced by local heat transfer from intruding sulfide liquid, which pervasively infiltrated country rocks on scales of tens to hundreds of metres beneath the orebodies. This additional heat source may be locally significant, despite being volumetrically limited. Retrograde P–T conditions were determined by garnet–biotite conventional thermobarometry of garnet-core–matrix biotite pairs (0.29–0.45 GPa and 550–640 °C). These P–T conditions suggest that the Nova-Bollinger country rocks cooled along a near-isobaric cooling path and reflect the effects of thermal decay following emplacement of voluminous magmas into the Fraser Zone during Stage I Albany–Fraser Orogeny.

The Kangâmiut dykes in West Greenland: markers of the tectono-metamorphic evolution of the southern Nagssugtoqidian orogen and its foreland

The general extent and structural evolution of the southern Nagssugtoqidian orogen of West Greenland were first described by Hans Ramberg who based much of his paper on the deformation of the regional Kangâmiut dyke swarm. The southern boundary is marked by a transition from undeformed, discordant dykes in the south to highly deformed dykes and host rocks to the north. Our analysis of the southern Nagssugtoqidian orogen and its southern foreland uses a comprehensive compilation of available data and covers the area from Sisimiut in the north to Alanngua, south of Maniitsoq. This represents almost the entire c. 200 km latitudinal extent of the Kangâmiut dyke swarm and encompasses the complete range of Nagssugtoqidian overprint on these dykes and their country rocks. South of Itillip Ilua (Itilleq), the structural and metamorphic overprints on the dykes exhibit a considerable range in both intensity and P–T conditions between and even within outcrops. In contrast, north of Itillip Ilua, the rocks show more systematic gradual increases in the degree of structural overprints and metamorphic grade, culminating in the Ikertooq thrust zone where granulite facies rocks are brought southwards over amphibolite facies rocks. Currently, available age data from the Nagssugtoqidian orogen permits the identification of two metamorphic episodes at c. 1850–1800 Ma and c. 1780–1720 Ma. These groups of metamorphic ages are supported by recent 40Ar–39Ar ages from dykes in the same area, which cluster at c. 1860 Ma and c. 1740 Ma, respectively. Albeit geographically sporadic, both age intervals support a subdivision of the Nagssugtoqidian structural and metamorphic overprints across the southern Nagssugtoqidian orogen and its foreland into two distinguishable temporal phases. Further geochronological investigations may well, however, find these two phases to be part of a tectonic continuum. For now, it is thought that the older event records south-directed thrusting over the foreland and concomitant loading of this crust, at least as far south as Maniitsoq. This c. 1860–1800 Ma crustal shortening and thrusting likely also closed a depositional basin located at the current latitude of Ikertooq, which could have formed during an early-orogenic extensional event that enabled and accompanied the c. 2035 Ma emplacement of Kangâmiut dykes. Up to 50–100 Ma later, a younger (c. 1780–1720 Ma) phase of shearing and thrusting mainly affected the Itillip Ilua – Ikertooq area and likely overprinted elements of the former event. This local younger overprint generated a separate trend of distinctly northward-increasing deformation and metamorphism.

A Study on the Singing Forms of Western Rock Music Integrating Classical Cultural Elements

Rock and roll is not only a form of music, but also a "attitude and philosophy of life". It is precisely because of this that rock and roll is different from general pop music, and there are various differences between Chinese and Western rock and roll. Rock music has many branches and complex forms. Its main styles include folk rock, art rock, psychedelic rock, Country rock, heavy metal, punk, etc. This article studies the singing forms of Western rock music that integrate classical cultural elements. People who listen to classical music "despise" the "vulgarity" of rock and roll, and people who play rock and roll "dislike" the "affectation" of classical music. These two music styles, which seem to never blend together, always maintain a proud attitude of respecting each other in their respective music culture circles. The extensive use of words with classical cultural elements and cultural imagery in rock music works can allow viewers to develop a sense of closeness and identification with the national essence reflected in the work from an aesthetic psychological perspective, and have a deeper understanding of the ideas expressed in the work.

Pit Slope Configuration for Open Pit Mining – A Case Study

To achieve stable pit wall slopes, it is imperative to obtain a fair knowledge of the rock mass characterisation before designing the pit. Insufficient knowledge of the competency of the country rock could lead to using unsupported slope configuration in the design process which can consequently lead to slope failure. In this study, the geomechnical properties of the Bremen-Nkosuo concession are analysed using Bieniawski’s classification scheme to determine the Rock Mass Rating (RMR) for defining safe pit slope configuration of the Nkosuo pit. The findings show that the rockmass are best described as ‘fair’ for the two main lithologies existing at the concession. Subsequently, localised adjustment factors are applied to the calculated RMR to arrive at Mining Rock Mass Ratings (MRMR). These MRMR values are correlated with 50 m fixed stack height and 1.2 safety factor to determine optimum Bench Slack Angle (BSA) of 54° and 57° for host sedimentary and granitic rocks respectively. For individual benches, optimum slope design configurations were 10 m, 800, and 6.6 m respectively for bench height, bench face angle and catch berm for metasedimentary rocks. Likewise, that for granitic formation were 10 m bench height, 800 face angle and 6.0 m catch berm width. These configurations are in conformance with mineral and mining regulations of Ghana. Slope stability assessment was performed which included Slope Mass Rating (SMR), Kinematic and Limit equilibrium analysis. From the analysis, multi-bench scale slope instability occurrence was found to be rare but single-double scale could be possible at the western wall of the planned pit with probability of failure of about 0.4. Presplit and trim shots perimeter blasting techniques are recommended to maintain the integrity of the final pit walls at certain areas.

A-type tonian granite in the northwest of the gavião block: Record of within-plate magmatism precursor to the continental rifting of the São Francisco paleoplate?

The Early Tonian period in the São Francisco Craton represents the development of an intracontinental rift across the northern and central Espinhaço regions, playing a crucial role in understanding the tectonic evolution of the Neoproterozoic Period within this craton and its adjacent orogens. This rift is associated with the breakup of the São Francisco paleoplate, marked by the formation of several rifts, some of which were aborted (e.g. Santo Onofre Rift), and magmatism associated. Field investigations, coupled with petrographic, lithogeochemical, and U–Pb geochronological data of the Serra do Meio Suite, elucidate key characteristics and offer significant insights into the Tonian tectonic evolution of the São Francisco Craton and its relationship with Santo Onofre Rift. The suite comprises metagranitoids exhibiting a mineralogical composition spanning from alkali-feldspar granites to granites and its petrogenesis and tectonic characteristics reveal an alkaline to calc-alkalic signature, exhibiting extreme ferroan geochemistry and indications of moderate to high oxygen fugacity. Pressure estimations suggest low-pressure conditions during magma crystallization, implying shallow crustal emplacement. The suite's evolution involves complex petrogenetic processes, likely influenced by fractional crystallization and contamination by country rocks from a quartz-feldspathic crustal source, as well as a possible contribution from juvenile tholeiitic mantle sources associated with rift environments, showcasing a signature typical of A1-type granite. The U–Pb SHRIMP age, determined as 930 ± 2 Ma (MSWD = 0.34), is interpreted as the crystallization age of the rock and allows us to connect it to the opening of the Santo Onofre-Macaúbas Rift in the São Francisco Craton.

Using Hg isotopes and trace elements to constrain the origin of the Ordovician Xiangquan Tl deposit, South China

Sediment-hosted stratiform sulfide (SHSS) deposits commonly contain large amounts of strategic metals, such as Tl, Hg, Cd, Ge and Ga, however, sources of these metals remain debate. Here we present new elemental and Hg isotopic data to constrain the metal sources for the Xiangquan Tl deposit (South China), a unique SHSS deposit with 250 t Tl at an average grade of 928 ppm. Stratiform pyrite orebodies are hosted in the Ordovician marine sedimentary succession of mostly limestone with minor mudstone. Pyrite, the dominant ore mineral, is rich in Hg (up to 1360 ppm). Bulk ores and pyrite separates from this deposit have significant variations of δ202Hg (−5.48 to −0.65 ‰) and Δ199Hg (−0.01 to 0.36 ‰). The Δ199Hg values are mostly positive, similar to those of seawater and marine sediments. Ore petrography, trace element enrichment coefficients, rare earth element patterns, Re-Os ages (478 ± 33 Ma) and S isotope compositions of pyrite indicate that Hg and other metals were derived from hydrothermal fluids venting at the seafloor. We suggest that the ore-forming fluids of the Xiangquan deposit were predominantly originated from large-scale circulation of evolved seawater, which leached Hg and Tl from the marine country rocks. These metal-rich fluids migrated upward, vented to the seafloor, and mixed with anoxic H2S-rich seawater to precipitate Hg- and Tl-rich pyrite. Our study highlights that the combined application of Hg isotopes and trace elements could be used to trace the sources of metals in hydrothermal ore deposits such as SHSS deposits.

Coupled Detachment Faulting and Hydrothermal Circulation at 49.7°E Southwest Indian Ridge Revealed by Seafloor Magnetism

AbstractDetachment faults at slow‐ and ultraslow‐spreading mid‐ocean ridges are essential for lithosphere‐hydrosphere interactions. However, their coupling with subseafloor hydrothermal circulation is poorly understood. Here, we investigated shallow hydrothermal fluid circulations in the active Dragon Horn detachment fault system of the Southwest Indian Ridge using combined near‐bottom magnetic anomaly data and rock magnetic analyses. We observed enhanced magnetic anomalies related to the high magnetization of basaltic country rocks and highly serpentinized (>80%) peridotite, whereas the hydrothermally altered basalts and sulfide deposits had weak magnetization and reduced magnetic anomalies. Therefore, the low‐magnetization zone was interpreted as a proxy for the hydrothermal fluid channel. The spatial distribution of the low‐magnetization zone revealed that the inactive Suye field expanded more than 500 m in diameter horizontally and ∼100 m vertically, and the active Longqi hydrothermal field had a >1 km fluid channel, both centered around the detachment fault fracture zone. Our results establish a robust link between detachment faulting and shallow hydrothermal circulation in which the detachment fault system provides critical fluid paths. Therefore, detachment tectonics are important elements for evolving seafloor hydrothermal fields along the Southwest Indian Ridge and possibly other slow‐ and ultraslow‐spreading mid‐ocean ridges globally.

Geological mapping and mineral exploration in Francistown region, north east Botswana from integration of ASTER TIR and aeromagnetic data

To map the region's geology and mineral exploration, this study looked at how remote sensing satellite imaging from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Thermal Infrared (TIR) sensor and aircraft geophysics may be used. The Francistown region forms part of the Zimbabwe craton, northeast of Botswana. It comprises felsic rock types, including granite and dry river sand, mafic-ultramafic rocks, and gneiss country rock. Based on their spectral emissivity responses, mafic-ultramafic units and quartz-rich units were discovered. These units are important targets for the discovery of mineral resources. The research region can be described as convoluted, fractured, intruded, metamorphosed, and ultimately worn from a geological and geophysical standpoint. Several remote sensing and aeromagnetic approaches were employed. The false color composite (FCC) and mineral indices using ASTER TIR data provided helpful information for delineating the area's geology and concentration of major-rock-forming minerals. The aeromagnetic data extracted lineaments well, and the Center for Exploration Target (CET) grid analysis system was very helpful. The CET Porphyry Analysis technique was applied in mapping near-circular porphyry features, which are good indicators of possible sites with high mineralization potential. The analysis of the lineament map shows that mineral deposits are mostly along the NE-SW direction. The results from the ASTER TIR and aeromagnetic enhancements indicate the occurrence of mafic-ultramafic rocks and felsic-rich units, which have been documented to host mineralization in the region. Porphyry structures are also mapped successfully along the general strike of the geological structure in the study area. The combination of these methods proved their success, and they can be used further in other regions of high mineral potential and lithological mapping for their cost efficiency and reliability.

The last gasp of the Rogaland Igneous Complex, Norway: a palaeopole for the 920 Ma Tellnes intrusion

SUMMARY The Rogaland Igneous Complex (RIC) in southern Norway intruded into Sveconorwegian granulite crust beginning ∼930 Ma. Three massif anorthosite bodies, Egersund–Ogna, Helleren and Åna-Sira, were intruded some 10 Myr later by the Bjerkreim–Sokndal layered intrusion. The Garsaknatt leuconorite and the ilmenite-rich Tellnes norite, one of the youngest rock in the complex at ∼920 Ma, intrude the anorthosite or nearby country rock. Magnetic mineralogy and palaeomagnetic studies carried out on the Tellnes norite, the Garsaknatt leuconorite and the surrounding Åna-Sira anorthosite, indicate the magnetization of all three bodies are dominated by hemo-ilmenite carrying the remanence as a thermochemical remanent magnetization, although magnetite is present in some samples. The three bodies yield steep negative inclinations with northwesterly declinations (Tellnes, I = −71.9°, D = 305.0°, α95 = 10.6°; Garsaknatt, I = −73.1°, D = 312.7°, α95 = 4.7°; and Åna-Sira, I = −81.2°, D = 326.3°, α95 = 6.7°). When combined with data from other bodies in the RIC, the older anorthosites have steeper inclinations, and higher palaeolatitudes, while the younger units have less steep inclinations and shallower palaeolatitudes by nearly 10°, indicating northward plate motion during cooling of the intrusions. Age of the remanence is difficult to determine precisely, however, best estimates are ∼910 Ma for the older anorthosites and ∼900 Ma for the younger intrusions. Although these differences are significant, a unified pole position (35.6° N, 215.1° E), combining all the 111 sites from the RIC, strongly supports the assumed position of southern Baltica in Rodinia at ∼900 MA.

Lithologic Relationship and Structural Features of Crystalline Rocks in Ekiti, Southwestern Nigeria: A Geological Report on the Basement Complex

This paper investigates and report field relationship and structural features of the basement rocks in Ekiti, southwestern Nigeria. Systematic geological mapping reveals that Ekiti is underlain by migmatite-gneiss, quartz schist, quartzite, granite, and charnockite. These lithologies were intruded by series of aplite, dolerite, and pegmatite dykes. Structural deformations attributable to polycyclic orogenic activities manifested in all the basement rocks. Migmatite and its gneissic subunits form the country rock and exhibits complex folds, abundant quartz veins and haphazardly emplaced dykes. Foliation and lineament are oriented N-S, strike values range between N5oE -N18oE with westerly dip of 72º to 84º. Quartzite and quartz schist exhibits distinctive joint and fracture system which runs parallel to one another while minor ones are oblique. The polymetamorphic terrain has older tectonic fabrics massively overprinted by younger ones. The gneiss units are porphyroblastic to granoblastic, xenoliths of varying sizes are embedded within the granite bodies indicating the magmatic liquid was forcefully injected into the country rock. The presence of parallel fractures in fine-grained granite indicate Ekiti area is a shear zone. The occurrence of dykes of heterogenous lithologies on single granite outcrop suggests pluton assemblage occur as discrete pulses during the intrusive phases. Contact relationship indicate age decreases from migmatite to granite-charnockite to microgranite while dykes are the youngest. Deformation, metamorphism, and intrusive phases are the dominant geodynamic features of Ekiti area while the regional and local changes in rocks resulted from environmental constraints of pressure, temperature, and fluid activity. The plutonic activities which accompanied orogenic events resulted in deformation which is genetically related to evolution and geodynamic setting of Ekiti area.